Films or sheets can be produced with a variety of plastic materials by a variety of processes (extrusion molding, stretch blow molding, etc.). Polycarbonates are widely used in a variety of molding and extrusion applications. Films or sheets formed from the polycarbonates must be dried prior to thermoforming. If the films and/or sheets are not pre-dried prior to thermoforming, thermoformed articles formed from the polycarbonates can be characterized by the presence of blisters that are unacceptable from an appearance standpoint.
Poly(1,4-cyclohexylenedimethylene) terephthalate (PCT), a polyester based solely on terephthalic acid or an ester thereof and 1,4-cyclohexanedimethanol, is known in the art and is commercially available. This polyester crystallizes rapidly upon cooling from the melt, making it very difficult to form amorphous articles by methods known in the art such as extrusion, injection molding, and the like. In order to slow down the crystallization rate of PCT, copolyesters can be prepared containing additional dicarboxylic acids or glycols such as isophthalic acid or ethylene glycol. These ethylene glycol- or isophthalic acid-modified PCTs are also known in the art and are commercially available.
One common copolyester used to produce films, sheeting, and molded articles is made from terephthalic acid, 1,4-cyclohexanedimethanol, and ethylene glycol. While these copolyesters are useful in many end-use applications, they exhibit deficiencies in properties such as glass transition temperature and impact strength when sufficient modifying ethylene glycol is included in the formulation to provide for long crystallization half-times. For example, copolyesters made from terephthalic acid, 1,4-cyclohexanedimethanol, and ethylene glycol with sufficiently long crystallization half-times can provide amorphous products that exhibit what is believed to be undesirably higher ductile-to-brittle transition temperatures and lower glass transition temperatures than the compositions revealed herein.
The polycarbonate of 4,4′-isopropylidenediphenol (bisphenol A polycarbonate) has been used as an alternative for polyesters known in the art and is a well known engineering molding plastic. Bisphenol A polycarbonate is a clear, high-performance plastic having good physical properties such as dimensional stability, high heat resistance, and good impact strength. Although bisphenol-A polycarbonate has many good physical properties, its relatively high melt viscosity leads to poor melt processability and the polycarbonate exhibits poor chemical resistance. It is also difficult to thermoform.
Polymers containing 2,2,4,4-tetramethyl-1,3-cyclobutanediol have also been generally described in the art. Generally, however, these polymers exhibit high inherent viscosities, high melt viscosities and/or high Tgs (glass transition temperatures) such that the equipment used in industry can be insufficient to manufacture or post polymerization process these materials.
In liquid crystal displays (LCDs) used for computers, laptops, television displays or other display systems, optical films or sheet material are commonly used to direct, diffuse, or retard or transmit with no interference the light and as support layers for polarizing films. For example, in backlit (backlight or sidelight) displays, brightness enhancement films use prismatic structures on the surfaces to direct light along a viewing axis (e.g., an axis normal to the display). Such films can enhance the brightness of the light viewed by the user of the display and can allow the system to consume less power in creating a desired level of on-axis illumination.
In Liquid Crystal Displays (LCD), it can be desirable to have diffusing components. Examples of the utility of diffusing components include, but are not limited to, masking artifacts, such as seeing electronic components located behind the diffuser film, improved uniformity in illumination and increased viewing angle. In a typical LCD display, diffusion of light is introduced into the backlight assembly by adding separate films (e.g., a stack) including a non-diffusing substrate, to which a highly irregular, diffusing surface treatment is applied or attached.
Additionally, attempts have been made to enhance properties of resins or resin compositions through the addition of fine particles, where such resins can be used as materials for optical uses such as LCDs, and touch panels. For example, as to optical resin sheets, such as light-diffusing sheets, there can be obtained by coating a surface of a predetermined base material with a resin composition prepared by mixing fine inorganic particles (e.g., titanium oxide, glass beads, and silica) or fine resin particles (made of, e.g., silicone resins, acrylic resins, or polystyrene) with a transparent resin as a binder. For light-leading plates, resin compositions have been prepared by adding resin particles (e.g., acrylic resins) into a transparent resin (e.g. polycarbonate) as a base material. However, there remains a need to generate diffuse light with out the added cost of separate films.
There are also reports of attempts to improve light diffusion properties of thermoplastic substrates, such as polyester or polycarbonate substrate, by the addition of inorganic minerals, e.g., BaSO4, a commonly used white pigment. Besides BaSO4, other minerals that may be used are aluminum oxide, zinc oxide (ZnO), calcium sulfate, barium sulfate, calcium carbonate (e.g., chalk), magnesium carbonate, sodium silicate, aluminum silicate, titanium dioxide (TiO2), silicon dioxide (SiO2, i.e., silica), mica, clay, talc, and the like in a range of up to about 25 weight percent. These minerals can cause formation of cavities or voids in the substrate, which can contribute to rendering the substrate more opaque due to multiple light scattering. However, the specifications applied to plastic sheets or films (substrates) in a number of homogeneous sheet or multi-wall sheet applications and optical applications may require, in some applications, that the substrates be substantially free of bubbles or cavities when thermoplastically processed, display minimal optical birefringence, have a low thickness tolerance or variation, low curvature, low thermal shrinkage, and low surface roughness.
Thus, there is a need in the art for LCD films or sheets comprising at least one polymer having a combination of two or more properties, chosen from at least one of the following: toughness, high glass transition temperatures, high impact strength, hydrolytic stability, chemical resistance, long crystallization half-times, low ductile to brittle transition temperatures, good color, and clarity, lower density and/or thermoformability of polyesters while retaining processability on the standard equipment used in the industry.